Semi-permeable membranes (e.g. reverse osmosis, nanofiltration, ultrafiltration, and microfiltration membranes) have a long history of use in separating solution components. Such membranes are a type of separation device that is able to retain certain substances while transmitting others. The components of the feed fluid that pass through the membrane are the “permeate” and those that do not pass through the membrane are the “retentate.” In practice, the permeate, the retentate, or both, may represent a desired product and may be used directly or be subjected to further processing.
Membranes have been widely used in a variety of water-based applications including water desalination, salt fractionation, and protein concentration. To a more limited extent, membranes have also been employed in non-water-based applications.
One limitation on the use of membranes in non-aqueous separations has been the need to “condition” the membrane prior to use. Typically, membranes are prepared in aqueous environments and they are preserved with water-based preservatives or by drying from glycerin. As a result of the residual glycerin, the membranes are typically not wettable with non-polar solvents or with non-polar feed mixtures. Consequently, it is often necessary to condition the membrane, for example, by contacting the membrane with a suitable intermediate solvent, prior to use in a non-aqueous separation process. This method has been used to convert water wet-membranes to a state useable with hexane-based oilseed miscella and with aromatic and aliphatic hydrocarbons. See International Patent Application Publication Numbers WO 0042138, and WO 0006526. Although this membrane conditioning technique has been used on a commercial scale, this method is expensive, time-consuming, and often requires the use of flammable and volatile organic compounds.
Additionally, K. D. Vos and F. O. Burris, Ind Eng Chem Prod Res Dev, 1969, 8, 84-89 report that water can be evaporated from certain specific modified cellulose acetate reverse osmosis membranes with no loss in desalination or physical properties by soaking the membranes in a surface active agent before drying. The properties of the dried membranes in non-aqueous media is not discussed.
In spite of the above reports, however, there is currently no simple, safe, cost-effective method to preserve a membrane for use in non-aqueous applications.